1. Field of the Invention
The present invention relates to an olefin (co)polymer composition. More specifically, the present invention relates to an olefin (co)polymer composition having high melt tension and excellent formability.
2. Description of the Prior Art
Olefin (co)polymers such as polypropylene and polyethylene or the like are widely used in a variety of molding fields because of excellent mechanical properties, chemical resistance and cost-effectiveness. Conventionally, the olefin (co)polymers have been generally produced by (co)polymerizing olefin by using a Ziegler-Natta catalyst, which is obtained by combining a transition metal catalyst composition such as titanium trichloride or titanium tetrachloride, or titanium trichloride or titanium tetrachloride supported on magnesium chloride, and an organic aluminum compound.
In recent years, on the other hand, a catalyst that is obtained by combining metallocene and aluminoxane, which is different from catalysts in the prior art, is used to (co)polymerize olefins to obtain olefin (co)polymers. The olefin (co)polymer obtained by using the metallocene-based catalyst has a narrow molecular weight distribution, and in the case of copolymers, comonomers are copolymerized uniformly. Therefore, it is known that more homogeneous olefin (co)polymers can be obtained. However, compared with olefin (co)polymers obtained by using a conventional catalyst, the olefin (co)polymers obtained by using the metallocene-based catalyst have a lower melt tension, so that they are not suitable for some uses.
In order to enhance the melt tension and the crystallization temperature, the following methods were proposed: a method of reacting polypropylene with an organic peroxide and a crosslinking assistant in a molten state (disclosed in Publication of Japanese Patent Application (Tokkai-Sho) Nos. 59-93711, 61-152754 or the like); and a method of reacting semi-crystalline polypropylene with a peroxide having a low degradation temperature in the absence of oxygen so as to produce polypropylene having free-end long branches and containing no gel (disclosed in Publication of Japanese Patent Application (Tokkai-Hei) No.2-298536).
Other methods for enhancing melting viscoelastic properties such as melt tension were proposed, such as a method of using a composition comprising polyethylenes or polypropylenes having different intrinsic viscosities or molecular weights, or producing such compositions by multistage polymerization.
Examples of such a method include a method in which 2 to 30 parts by 10 weight of ultra high molecular weight polypropylene are added to 100 parts by weight of ordinary polypropylene and extrusion is performed in a temperature range from a melting point to 210.degree. C. (disclosed in Japanese Patent Publication (Tokko-Sho) No. 61-28694), a method using multistage polymerization to obtain an extrusion sheet formed of two components of polypropylene having different molecular weights and an intrinsic viscosity ratio of at least 2 (disclosed in Japanese Patent Publication (Tokko-Hei) No. 1-12770), a method of producing a polyethylene composition formed of three types of polyethylene having different viscosity average molecular weights comprising 1 to 10 wt % of high viscosity average molecular weight polyethylene by melting and kneading or multistage polymerization (disclosed in Japanese Patent Publication (Tokko-Sho) No. 62-61057), a method for polymerizing polyethylene in which 0.05 to 1 wt % or less of ultra high molecular weight polyethylene having an intrinsic viscosity of 20 dl/g or more is polymerized by multistage polymerization (disclosed in Japanese Patent Publication (Tokko-Hei) No. 5-79683), a method for polymerizing polyethylene in which 0.1 to 5 wt % of ultra high molecular weight polyethylene having an intrinsic viscosity of 15 dl/g or more is polymerized by multistage polymerization in a polymerization reactor having a special arrangement by using a highly active titanium catalyst composition preliminarily polymerized with 1-butene or 4-methyl-1-pentene (disclosed in Japanese Patent Publication (Tokko-Hei) No.7-8890) or the like.
Furthermore, Japanese Patent Application Publication (Tokkai-Hei) No. 5-222122 disclosed a method for producing polypropylene having a high melt tension by polymerizing polypropylene by using a preliminarily polymerized catalyst obtained by preliminarily polymerizing compounds of ethylene and polyene with a supported Ti solid catalyst composition and an organic aluminum compound catalyst composition. Japanese Patent Application Publication (Tokkai-Hei) No. 4-55410 disclosed a method for producing linear low density polyethylene (LLDPE) having a high melt tension by using a preliminarily polymerized catalyst containing polyethylene having an intrinsic viscosity of 20dl/g or more obtained by preliminarily polymerizing ethylene alone with the same catalyst compositions as above.
Furthermore, the following methods were proposed in order to enhance a melt tension in the case where a metallocene catalyst type is used: a method of using a catalyst comprising a silica support containing at least 1.0 wt % of water, metallocene, methylaluminoxane and triisobutyl aluminum (as disclosed in Japanese Patent Application Publication (Tokkai-Hei) No.5-140224); a method of using two types of metallocene as catalyst components (as disclosed in Japanese Patent Application Publication (Tokkai-Hei) Nos. 5-255436, 5-255437 and 6-206939); and a method of using montmorillonite as a metallocene catalyst type (as disclosed in Japanese Patent Application Publication (Tokkai-Hei) Nos. 7-188317 and 7-188336).
In the above-mentioned various compositions and the production methods in connection with the catalyst types in the prior art, the melt tension of polyolefin is enhanced to some extent under measurement conditions at 190.degree. C. However, other problems still remain unsolved with respect to the melt tension under use conditions at 200.degree. C. or more, a residual odor caused by the crosslinking assistant, the crystallization temperature, the heat stability of properties other than the melt tension, or the like.
Furthermore, although the proposed methods in connection with the metallocene catalyst type provide an improvement of the melt tension of polyolefin under measurement conditions at 190.degree. C., it is still desired to further improve the melt tension under use conditions at 200.degree. C. or more.